It's a weather pattern caused by the return of the marine layer to Southern California and means cool and cloudy mornings for the region.
You most likely know it from the early morning clouds and fog that roll in and give way to afternoon sunshine.
This May gray pattern happens when cool and moist air is trapped near the surface due to a temperature inversion.
It is seen in Southern California thanks to the contrast of the cool temperatures of the Pacific Ocean and warmer air above. It results in fog and clouds along the coast, but it's not uncommon for that cloud cover to extend inland.
Just how far that marine layer extends inland depends on several factors, including how deep it is. For example, at 1,000 feet, cloud cover is probably visible along the coast, but if it extends up to 5,000 feet that cloud cover could be seen pushing inland and bringing a chance for light drizzle.
Some years, we see it stick around much longer and turn into "June gloom" or even "No-sky July." It's nearly impossible to pinpoint exactly how long it will stick around, but since sea surface temperatures are warmer than usual, that could mean we don't see it stick around as long this year.